In marine drives, both dog clutches and friction clutches have been employed However, dog clutches provide an undesirable jerk when engaged, particularly when the engine is running at high speeds. On the other hand, friction clutches require considerable clamp force and are not well suited to remote control. Many mechanically actuated clutches, as compared to hydraulic or magnetically actuated clutches, utilize a single clutch element or combination of clutch elements, such as friction clutch packs, cone clutches and/or ball screw mechanisms.
The combination of a cone clutch with a friction clutch pack has been commonly used in marine transmissions where a pair of opposite friction packs are combined with a cone clutch for each mechanism, one clutch pack acting to actuate the forward drive and the other clutch pack actuating the reverse drive; conventionally the forward/reverse drive is manually actuated from the exterior of the transmission. In a construction where a numerically high mechanical advantage clutch loading mechanism is used, which is totally independent of the torque acting on and passing through the clutch, the clutch clamping mechanism usually has a very limited travel, and there are problems in accommodating the take-up of the clearances and the wear allowance for the clutch. This limited travel restriction is present because the ratio of the movement between the clutch operating member and the clutch clamping member is usually inversely proportional to the ratio of the mechanical advantage. Consequently, in-service adjustments or automatic wear compensators are required, which can add considerable expense to the clutch.
The U.S. Pat. No. 4,287,973 to Eichinger et al discloses a shiftable friction clutch for gear drives wherein a combination clutch utilizes interleaved compressible fiction rings or overlapping conical rings and a selectively operable structure effecting engagement of either clutch device. The selectively operable device includes a roller and ramp structure between the driving and driven parts responding to torque initially applied to the driving member for generating an axial force which is then applied to the clutch pack compressing structure.
The British Patent Application GB No. 2170870A discloses an embodiment of power shift clutch which includes a clutch loading device in a boat gearbox, the device having a ball screw mechanism in combination with multiple friction plate power drive clutches at the respective ends of the transmission structure. However, the ball screw helix angle is self-releasing for the clutch, and the shift mechanism requires locking pins and springs. The pins and springs are the origin of the force required for a cone clutch which is also inserted in the clutch as part of the actuating mechanism. The detent force caused by the pins and springs subtract from the force which passes through the clutch pack and reduces the efficiency of the design. Also, for marine applications, excess torque could occur if the propellor driven by the transmission were to strike an underwater object. This excess is controlled by slippage which occurs initially at the cone clutch and then at the clutch pack. Further, the torque capacity of this mechanism is limited depending on the point where the cone clutch begins to slip.
The present invention provides a ball screw actuated clutch mechanism which is used in combination with a pair of oppositely disposed clutching mechanisms for the forward and reverse directions.